Accordingly, the invention does not relate to probes the photosensitive element of which (pyrometer or other) remains outside the oven, at one end of either a rigid and straight rod, the other end of which accommodates a deflecting mirror situated in front of an open window facing the walls of the flues (U.S. Pat. No. 3,577,784), or of a flexible element shielding one or more optical fibres terminating laterally at the other end, within the oven (U.S. Pat. No. 4,402,790).
In a first embodiment of a known probe of the type according to the invention, the optical pyrometer is enclosed in a jacket which is cooled by internal circulation of water or the like. This jacket constitutes the measuring head and is placed at the end of a rigid rod manipulated by a rack system. This embodiment forms an assembly which is heavy, bulky, difficult to manipulate and expensive and which is, in addition, liable to interfere with the measurement as a result of the presence of a large thermal mass.
In a second type of known probe, the rigid rod has been replaced by a flexible metal tube which can be coiled and which conducts the cooling water to the head. This system has the disadvantage of being fragile, in particular due to the risks of breaking out the flexible tube.
Until now, by reason of the high temperatures prevailing within the flues and in view of the fact that optical pyrometers do not withstand a temperature exceeding 60.degree. C., those skilled in the art have always considered it essential to provide cooling of the head by internal circulation of fluid (in general, water).
The inventors have recognized that it was possible to dispense with such cooling by circulation of water which, if effective, nevertheless created serious restraints. They have discovered that it is possible to design a simple apparatus permitting successive operations of inspection of each flue which are sufficiently rapid to dispense with an elaborate cooling system.